Search for Bs0 → μ+μ- and B0 → μ+μ- Decays with CDF II

A search has been performed for B-s(0) --> mu(+)mu(-) and B-0 --> mu(+)mu(-) decays using 7 fb(-1) of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron collider. The observed number of B-0 candidates is consistent with background- only expectations and yields an upper limit on the branching fraction of B(B-0 --> mu(+)mu(-)) < 6.0 X 10(-9) at 95% confidence level. We observe an excess of B-s(0) candidates. The probability that the background processes alone could produce such an excess or larger is 0.27%. The probability that the combination of background and the expected standard model rate of B-s(0) --> mu(+)mu(-) could produce such an excess or larger is 1.9%. These data are used to determine B(B-s(0) --> mu(+)mu(-)) = (1.8(-0.9)(+1.1)) x 10(-8) and provide an upper limit of B(B-s(0) --> mu(+)mu(-)) < 4.0 x 10(-8) at 95% confidence level.